As is the case with most things in medicine, the answer is - it could be any of those things. By definition, an aneurysm is a dilation of a blood vessel due to weakening of the vessel wall. The eventual dilation is a mechanical failure, but your question is how does the vessel arrive at that weakened state. There are many possibilities. The most simple is hemodynamic stress (think hypertension). Aneurysms are known to occur at branch points where the wall is structurally weakest (ex. ACA in the brain). Pathologically, any damage to the innermost layer, the endothelium, promotes an inflammatory response, which can further weaken the wall via enzymes such as proteases. All of these processes occur in conjunction, so it is not simply 'inflammation or mechanical stress', it is commonly all of them at once. There are also genetic and autoimmune diseases that can cause the pathological breakdown of the vessel walls. Connective tissue disorders such as Marfans or Ehlers-Danlos syndrome are characterized by the inability to produce adequate structural proteins, while autoimmune disorders can lead to auto-antibodies that target your own connective tissue, breaking it down. In short, all of the processes you mentioned are almost always present in some way. The 'main' cause is impossible to say, but all of the various processes eventually funnel down into mechanical weakening of the vessel wall that cannot withstand the hydrostatic force of your blood pressure.
